1. Field of the Invention
This invention relates to offshore platforms for oil and gas exploration and production. More particularly, it relates to vertical risers used on such facilities to connect to wellheads on the seafloor.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
Conduits are needed to transfer materials from the seafloor to production and drilling facilities atop the water's surface, as well as from the facility to the seafloor. Subsea risers are a type of pipeline developed for this type of vertical transportation. Whether serving as production or import/export vehicles, risers are the connection between the subsea field developments and production and drilling facilities.
There are a number of types of risers, including attached risers, pull tube risers, steel catenary risers, top-tensioned risers, riser towers and flexible riser configurations, as well as drilling risers.
A top-tensioned riser is a vertical riser system that terminates at or near a point on the seafloor directly below the facility. Top-tensioned risers are often used on TLPs and spars. Although moored, these floating facilities are able to move laterally with the wind and waves. Because the rigid risers are fixed to the seafloor, such movement causes vertical displacement between the top of the riser and its connection point on the facility. There are two solutions for this issue. A motion compensator can be included in the top-tensioning riser system that keeps constant tension on the riser by expanding and contracting with the movements of the facility. Alternatively, buoyancy cans, can be deployed around the outside of the riser to keep it afloat. Then the top of the rigid vertical top-tensioned riser is connected to the facility by flexible pipe, which is better able to accommodate the movements of the facility.
U.S. Pat. No. 4,733,991 to Myers describes an adjustable riser top joint for connecting an offshore subsurface well to a deck-mounted welltree. A first plurality of generally annular protrusions on the riser top joint section affords a plurality of connecting points for the wellhead tree using either a unitary or a split collar type attachment. A second plurality of protrusions positioned below deck afford a second plurality of connecting points for riser tensioning means that may also, preferably, be attached using either a unitary or a split collar. The generally annular protrusions are formed as a continuous spiral groove on an external surface of the riser section in a first embodiment and as a series of generally cylindrical protrusions of equal length and spacing in a second embodiment.
PCT Publication No. WO 2012044928 entitled “Riser System for a Slacked Moored Hull Floating Unit” describes a riser tensioning system with an individual riser unit on an oil or gas platform. The platform has a traveling trolley structure with at least one trolley bearing and a centralizer. The trolley bearing is coupled to at least one guide rail configured to allow vertical movement of said traveling trolley structure. The traveling trolley structure is coupled to a riser collar configured to support a top of the riser. The riser tensioning system has at least one cylinder coupled to the traveling trolley structure on one end and secured on an opposite end such that the cylinder is adapted to push or pull the traveling trolley structure vertically.
U.S. Pat. No. 8,021,081 describes a tensioner system for a top-tensioned riser in a floating platform that includes a hydro-pneumatic tensioner assembly resiliently mounted to the floating platform, and a riser support conductor surrounding the riser coaxially, wherein the support conductor conveys a pull-type tensional force from the hydro-pneumatic tensioner assembly to the riser through a riser conductor coupling assembly that engages the tensioner assembly and the riser support conductor to convey the tensional force. A riser tension joint support assembly conveys the tensional force from the riser support conductor to a riser tension joint on the riser. The tensioner assembly compensates for relative platform motion including pitch, heave, and yaw. Also a reactive load assembly is mounted to the platform and reacts to a two-point dynamic bending moment imposed on the riser support conductor, while resisting riser support conductor rotation.
U.S. Pat. No. 7,588,393 describes a method for supporting top-tensioned drilling and production risers on a floating vessel using a tensioner assembly above the waterline of the vessel. The method can include attaching at least one hydraulic cylinder on a first end to a first position on a floating vessel and on a second end to a tension frame below the first position. The next step of the method can be forming a fluid connection between the at least one hydraulic cylinder and at least one primary accumulator.
U.S. Pat. No. 7,654,327 describes a tensioner assembly for moveably carrying a conductor that communicates from a wellhead to a piece of well access equipment on a rig. The tensioner assembly includes a support frame, at least one hydraulic cylinder connected to the support frame, and at least one primary accumulator in fluid communication with the hydraulic cylinder.
U.S. Publication No. 2007/0258775 describes a means for applying a controlled tension to a top tension riser on a floating offshore structure. A mechanical tensioner device and a buoyancy can are combined to apply controlled tension to an individual riser or group of risers supported by a floating offshore structure. The buoyancy can applies static tension force on the riser(s). The mechanical tensioner applies additional tension force to assist the can in limiting the stroke of the riser as the supporting structure is displaced from its nominal position.
U.S. Publication No. 2012/0292042 describes a ram tensioner system with a deck mountable frame having an upper portion and a lower portion connected by a plurality of cylinder sleeves and at least one guide post sleeve. A guide post engages the guide post sleeves and an individually replaceable modular cylinder is in each cylinder sleeve along with at least one individually removable seal gland that is lubricated by a hydraulic power unit, and a slidable rod engaging each of the cylinders. The slidable rods can be attached to the tension deck with a tension ring to engage a riser and provide movable tension to the riser.
U.S. Pat. No. 7,632,044 to Pallini et al. describes a ram style tensioner with a fixed conductor and a floating frame. The riser tensioner for an offshore floating platform has a frame stationarily mounted to the upper portion of the riser. Pistons and cylinders are spaced circumferentially around the riser and connected between the frame and the floating platform. A tubular guide member is mounted to the floating platform for movement in unison in response to waves and currents. The riser extends through the guide member. A guide roller support is mounted to and extends downward from the frame around the guide member. At least one set of guide rollers is mounted to the guide roller support in rolling engagement with the guide member as the guide member moves in unison with the platform.
U.S. Pat. No. 8,123,438 to Pallini et al. describes a ram-style riser tensioner that includes a frame configured to be fixedly attached to the riser; plural cylinder assemblies spaced around the riser, each cylinder assembly having a cylinder and a piston configured to slidably move inside the cylinder, the piston being configured to connect to the frame; a guide roller support stationarily mounted to and extending from the frame; at least one bearing fixedly attached to the guide roller support; and a guide member configured to be in rolling engagement with the at least one bearing as the cylinder moves relative to the frame.
A paper entitled Riser Tensioners for a TLP Application by F. H. MacPhaiden and J. P. Abbot presented at the 17th Annual Offshore Technology Conference in Houston, Tex., May 6-9, 1985, (OTC 4985) describes ram-type production and drilling riser tensioners which include a support structure that transmits the tension loads to the platform, ram-accumulator assemblies that generate the required forces needed for tensioning the riser, an upper support ring that acts as an interface between the hydraulic rams and the riser string, retractable latches that provide a suitable landing shoulder for the riser tension sub, and guide rails that transmit horizontal and torsional riser loads into the platform.